This year, and in every year in the foreseeable future, 17,000 Americans will develop primary brain cancers. Of these, the most common tumor is astrocytoma. Of the 8,000 people to be diagnosed with astrocytoma, all will eventually die of their disease. The vulnerability of the astrocyte to transformation lies in its ability to re-enter the cell-cycle at any point in the life-time of an organism. In spite of the ability to proliferate, astrocytes are kept mostly quiescent, except in response to disease or trauma, where there is a concomitant neuronal loss. A number of years ago, we and others demonstrated that astrocyte proliferative control is effected by contact with neuronal membranes, although the precise molecular mechanism by which neurons exert this control has remained elusive. My laboratory has recently identified a receptor on the surface of the astrocyte, CD8 1, which is absolutely required for neuron-induced astrocyte cell-cycle arrest (see Preliminary Data, and appended manuscript for details). In this application, we propose a series of biochemical, molecular and cell biological experiments aimed at the identification and characterization of neuronal CD8 1 binding proteins. In addition, we will begin to query the astrocytic signaling mechanism following neuronal binding. Our findings for the requirement for astrocyte expressed CD8 1 for neuron-induced growth control takes on added significance, based on our observation that all of the astrocytoma cell lines we have examined to date have absolutely down regulated CD8 1 protein and message. To determine if CD8 1 expression can rescue the ability of these cells to respond to neuron-induced by cell-cycle arrest, we have expressed a CD8 1 -GFP fusion construct in the astrocytoma cell lines. The transfectants will be assayed for in vitro neuronal responsiveness, and in vivo tumor progression and metastasis in nude mice. In parallel with these experiments, we will continue a series of affinity chromatographic purification of neuronal membrane proteins that are active inhibitors of astrocyte proliferation. While we hope and anticipate that our two lines of inquiry on neuronal mediators of astrocyte growth control will converge, it is quite possible that we will identify separate, redundant mechanisms that are involved in maintaining CNS numerical homeostasis.